1. Field of the Invention
This invention relates generally to telecommunications devices, and, more particularly, to a method and apparatus employing a cutback probe. Specifically, the present invention relates to a method and apparatus employing a politeness and off-hook cutback probe.
2. Description of the Related Art
In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire, bi-directional communication channel. The Plain Old Telephone System (POTS), designed primarily for voice communication, provides an inadequate data transmission rate for many modem applications. To meet the demand for high-speed communications, designers have sought innovative and cost-effective solutions that take advantage of the existing network infrastructure. Several technological advancements have been proposed in the telecommunications industry that make use of the existing network of telephone wires. One of these technologies is the xDSL technology. DSL technology uses the existing network of telephone lines for broadband communications. An ordinary twisted pair equipped with DSL interfaces can transmit videos, television, and high-speed data.
DSL technologies typically leave the POTS service undisturbed. Traditional analog voice band interfaces use the same frequency band, 0-4 Kilohertz (kHz), as telephone service, thereby preventing concurrent voice and data use. A DSL interface, on the other hand, operates at frequencies above the voice channels from 100 kHz to 1.1 Megahertz (MHz). Thus, a single DSL line is capable of offering simultaneous channels for voice and data.
DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. Certain DSL systems provide a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of about 1.5 Megabits per second (MBPS). The transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS) transfer rate.
One popular version of the DSL technology is the Asymmetrical Digital Subscriber Line (ADSL) technology. The ADSL standard is described in ANSI T1.413 Issue 2, entitled, xe2x80x9cInterface Between Networks and Customer Installationxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interface,xe2x80x9d the most recent revision of which as of the filing date of this specification is incorporated herein by reference in its entirety.
ADSL modems use two competing modulation schemes: discrete multi-tone (DMT) and carrierless amplitude/phase modulation (CAP). DMT is the standard adopted by the American National Standards Institute. The standard defines 256 discrete tones. Each tone represents a carrier signal that can be modulated with a digital signal for transmitting data. The specific frequency for a given tone is 4.3125 kHz times the tone number. Tones 1-7 are reserved for voice band and guard band (i.e., tone 1 is the voice band and tones 2-7 are guard bands). Data is not transmitted near the voice band to allow for simultaneous voice and data transmission on a single line. The guard band helps isolate the voice band from the ADSL data bands. Typically, a splitter may be used to isolate any voice band signal from the data tones. Tones 8-32 are used to transmit data upstream (i.e., from the user), and tones 33-256 are used to transmit data downstream (i.e., to the user). Alternatively, all the data tones 8-256 may be used for downstream data, and upstream data present on tones 8-32 would be detected using echo cancellation. Because more tones are used for downstream communication than for upstream communication, the transfer is said to be asymmetric.
Through a training procedure, the modems on both sides of the connection sense and analyze which tones are less affected by impairments in the telephone line. Each tone that is accepted is used to carry information. Accordingly, the maximum capacity is set by the quality of the telephone connection. The maximum data rate defined by the ADSL specification, assuming all tones are used, is about 8 MBPS downstream and about 640 KBPS upstream.
ADSL modems generally employ some mechanism for regulating the power levels to reduce cross talk and non-linear effects from high signal levels. Two mechanisms of regulating power levels are politeness cutback and off-hook cutback. Politeness cutback is generally a cutback applied to prevent overload of an analog-to-digital (A/D) converter of a receiver. The A/D converter of the receiver is likely to be overloaded, for example, when the subscriber loop is short and the attenuation of the line is relatively small. The off-hook cutback is primarily utilized to provide reduction in signal level to remove non-linear elements.
Existing communications systems that employ some form of politeness cutback are generally unreliable, inefficient, and unnecessarily constrictive. Existing communications systems, for example, typically rely on open loop systems to determine politeness cutback, which means that there is no direct communication of power levels. Thus, for instance, a device may measure upstream receiver power to indirectly infer the downstream power required for transmission. This method has several shortcomings. First, such a method requires a priori power level and thereby may be unnecessarily constrictive, and perhaps inefficient. Second, such a method may be unreliable since it employs an indirect approach for determining the required downstream power. This indirect approach is especially prone to inaccuracies in instances where different wires or frequencies are utilized for upstream and downstream communications.
Prior communications systems employing a method to determine off-hook cutback also tend to be unreliable, inefficient, and unnecessarily constrictive. Such systems transmit maximum power, estimate existing distortion levels, and then determine the cutback needed to reduce the distortion to an acceptable level. This method of determining cutback, however, has several shortcomings. First, such a method virtually ensures a strong signal on the line, causing interference to other services, thereby producing the very result the off-hook cutback is intended to avoid. Second, such a method requires a priori power level and therefore may be unnecessarily constrictive, as well as inefficient.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
One aspect of the present invention is seen in a method that includes establishing a pathway from a first transceiver to a second transceiver, determining a power level at the second transceiver, and determining whether the power level at the second transceiver is at an acceptable level over the pathway. The method also includes adjusting a power level at the first transceiver in response to determining whether the power at the second transceiver is at the acceptable level.
Another aspect of the present invention is seen in an apparatus that includes a first and second logic. The first logic is capable of establishing a pathway with a transceiver. The second logic is capable of determining a power level at the transceiver, determining whether the power level at the transceiver is at an acceptable level over the pathway, and adjusting a power level of the apparatus in response to determining whether the power at the transceiver is at the acceptable level.